The present invention is directed generally to low dielectric constant films and more particularly to low dielectric constant films prepared by a sol-gel method using hyperbranched polycarbosilane as a precursor.
The growth of integrated circuit (IC) technology is primarily based on the continued scaling of devices to ever-smaller dimensions. Smaller devices provide higher packing density and higher operating speed. In the ultra-large-scale integration (ULSI) era, the millions, and soon to be billions, of transistors on a chip must be interconnected to give desired functions. As minimum device features shrink below 0.25 microns, the increase in propagation delay, cross-talk noise and power dissipation of the interconnect become limiting factors. It is, therefore, desirable to reduce the interconnect capacitance in order to maintain the trend of reduced delay time, reduced power consumption and reduced noise for future scaled devices. Capacitance is directly proportional to dielectric constant (k).
Currently the most common semiconductor dielectric is silicon dioxide, which has a dielectric constant of about 4.0. Thus there is substantial interest in materials with low dielectric constants that can replace SiO2-based insulators as inter layer dielectrics (ILD), as discussed in W.W. Lee and Paul S. Ho, xe2x80x9cLow-Dielectric-Constant Materials for ULSL Interlayer Dielectric Applicationsxe2x80x9d, MRS Bulletin, 19, October, (1997). However, these materials should also meet many other criteria besides low dielectric constant. Table I lists some the requirements for low k dielectrics, as discussed in Laura Peters, xe2x80x9cPursuing the Perfect Low-k Dielectricxe2x80x9d, Semiconductor International, Vol. 21, No. 9, (1998).
Many materials have been proposed as candidates for low-k ILDs. These materials fall into three categories: organic, inorganic, and hybrid materials (generally organosiloxanes). The two deposition techniques being most strongly investigated are chemical vapor deposition (CVD) and spin-on deposition. Spin-on deposition lends itself to a much wider class of materials and deposition conditions are much easier to establish, as discussed in L. Peters, xe2x80x9cLow-k Dielectrics: Will Spin-On or CVD Prevail?xe2x80x9d, Semiconductor International, Vol. 23, No. 6, (2000).
Due to their hydrophobic properties and reduced polarizability, organic materials typically have a lower dielectric constant at equivalent porosities than do inorganic materials. Most spin-on organic polymers are significantly different from spin-on glass because moisture does not evolve during curing, and they have superior crack resistance, as discussed in P. Nunan, Yield Management Solutions, 17, Spring, (2000). However, with many materials in this category, thermal stability is a primary concern. In the temperature regime around 425xc2x0 C., these organic polymers typically undergo severe outgassing and have already begun to decompose, as discussed in C. B. Case, A. Kornblit, M. E. Mills, D. Castillo, R. Liu, Mat. Res. Soc. Symp. Proc. 443, 177, (1997).
In contrast, inorganic materials are generally integrated more easily into existing semiconductor manufacturing processes because they retain a SiO2-like matrix. Examples of inorganic materials are silica xerogels and aerogels, as described in A. Jane, S. Rogojevic, S. V. Nitta, V. Pisupatti, W. N. Gill, P. C. Wayner, J. L. Plawsky, Mat. Res. Soc. Symp. Proc., 565, 29, (1999) and in M. Jo, H. Park, D. Kim, S. Hyun, S. Choi, J. Paik, J. Appl, Phys. 82(3), 1299, (1997), respectively. In addition, these inorganic materials are very thermally stable due to the strong Sixe2x80x94O bonding. But strong Sixe2x80x94O bonding also brings high polarizability and brittleness to the materials. Nanoporous silica, a porous SiO2 network structure, has been developed to lower the dielectric constant while keeping its high thermal stability, as described in L. Peters, xe2x80x9cIndustry Divides on Low-k Dielectric Choices.xe2x80x9d Semiconductor International, May (2001) and T. Ramos, K. Roderick, A. Maskara, D. M. Smith, Mat. Res. Soc. Symp. Proc. 443, 91, (1997). However, the ability to control the pore structure still needs to be improved.
Organic/inorganic hybrid materials are attractive since they may gain some advantages from both the organic and inorganic regimes. In particular, silsesquioxanes, with the empirical formula (RSiO3/2))x, where R is hydrogen or an organic group, have attracted much attention as promising candidates for low dielectric constant materials, as described in H. Lee, E. K. Lin, H. Wang, W. Wu, W. Chen, E. S. Moyer, Chem. Mater. 14, 1845, (2002) and L. Lee, W. Chen, W. Liu, J. Poly. Sci. Part A: Polymer Chemistry, Vol. 40, 1560, (2002).
Various alkoxysilanes, including tetraethoxy silane [Si(OEt)4], RSi(OR)3 (R=H and alkyl or aryl), and (Rxe2x80x2O)3Sixe2x80x94Rxe2x80x94Si(ORxe2x80x2)3 (R=ethylene, phenylene, and various organic linking groups), have been used as precursors in the sol gel preparation of silica and organosiloxane films. Some of these films have been found to show relatively low dielectric constants, as discussed in S. Sugahara, T, Kadoya, K. Usami, T. Hattori, M. Matsumura, J. Electrochem. Soc. 148(6), (2001) and S. Sugahara, K. Usami, M. Matsumura, J. Appl. Phys., Part 1 38(3A), 1428, (1999).
Polycarbosilanes, in which Sixe2x80x94C bonds form the backbone of the polymer, can be viewed as a xe2x80x9chybridxe2x80x9d between the purely organic and inorganic polymers. They can potentially combine the advantages of these two classes of polymers. The Sixe2x80x94C bond is essentially non-polar (the silicon atom has a slight positive charge), which lessens the opportunity for electrophilic or nucleophilic attack on the Sixe2x80x94C bond, as well as lowering the bond dipole contribution to the overall dielectric constant relative to the Sixe2x80x94O bonds in silica. This makes the Sixe2x80x94C bond the most chemically inert bond that a silicon atom can form.
In U.S. Pat. No. 5,602,060, incorporated by reference herein, a solution of a specified polycarbosilane is applied in a solvent onto a substrate having electrically conductive components fabricated therein. The coated layer of the polycarbosilane is then cured in an oxidizing atmosphere to convert the polycarbosilane layer to a silicon oxide layer. The resulting silicon oxide layer has a planarized surface and shows no cracking. However the dielectric constant is not low enough to meet the ILD dielectrics requirements. Moreover, the final layer obtained after the atmospheric curing is basically silica and thus has no organic character remaining.
U.S. Pat. No. 6,255,238, incorporated by reference herein in its entirety, describes how hydridopolycarbosilanes can be subjected to heating under controlled conditions to generate certain cross-linked polyorganosilicon films having low dielectric constants of between 2.4 and 3.8. The heating can be done by a thermal source, an electron-beam, UV light and any other high-energy source. The patent indicates that baking the gelled film at several different sequentially elevated temperatures in air prior to curing the gelled film in an inert ambient decreases the dielectric constant to between 2.4 and 3. However, a dielectric constant below 2.4 could not be obtained by the method disclosed in this patent.
An alkoxy-substituted, hyperbranched polycarbosilane can be synthesized by Mg-induced coupling of Cl3SiCH2SiCl3, followed by the substitution of Cl by the methoxy or ethoxy group, as described in Q. Liu, W. Shi, F. Babonneau and L. V. Interrante, Chem. Mater., 9, 2434, (1997). This hyperbranched polymer has a complex structure with a distribution of Si environments that range from CH2Si(OR)3 to (CH2)4Si and an average molecular weight (Mn) of ca. 600-1000 AMU. The Si(OR)n groups can be subjected to hydrolysis and condensation via the xe2x80x9csol-gelxe2x80x9d process to give, after drying and complete condensation, an inorganic/organic network polymer having the compositional formula, [Si(O)CH2]n that contains both Sixe2x80x94Oxe2x80x94Si and Sixe2x80x94CH2xe2x80x94Si bridging groups, and under certain circumstances at least, was found to have high internal porosities, and to resist weight loss and other chemical changes to almost 600xc2x0 C., as discussed in G. D. Soraru, Q. Liu, L. V. Interrante and T. Apple, Chem. Mater., 10, 4047, (1998).
A Ph.D. thesis of Qi Liu, granted by the Chemistry Department, Rensselaer Polytechnic Institute, Troy, N.Y., July 1998 describes efforts to prepare such inorganic/organic network polymer films. Although the films were successfully obtained by Ms. Liu by using sol-gel processing these films were quite thin (1,788 Angstroms or less) and had refractive indexes ranging from 1.43 to 1.36, which is only slightly lower than those of silica films, suggesting loss of the porosity during film processing and a dielectric constant of above 2.5.
A preferred embodiment of the present invention provides a hybrid organic/inorganic organosilicon networked polymer material having a compositional formula [Si(O)CH2]n and a dielectric constant of less than 2.4.
Another preferred embodiment of the present invention provides a method of making a hybrid organic/inorganic film, comprising providing an alkoxy substituted hyperbranched polycarbosilane precursor material, dissolving the precursor material in a solvent to form a solution, adding a catalyst to the solution, coating the solution over a substrate to form a gelled film, and curing the gelled film to remove the solvent from the gelled film, such that the hybrid organic/inorganic film with a dielectric constant of less than 2.5 is formed.